chore!: Enable nullable reference types (#253)

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## Enable nullable reference types
<!-- add the description of the PR here -->

- This PR enables the nullable validation and treats warnings as errors.

### Related Issues
<!-- add here the GitHub issue that this PR resolves if applicable -->

Fixes #208 

### Notes
<!-- any additional notes for this PR -->
This PR turns on the nullable checks for the dotnet checks. This gives
us a better and safer codebase since it checks for potential null
exceptions.

### Breaking changes
While this PR won't require changes to the exposed API, it might show
some errors to the clients consuming it.

---------

Signed-off-by: André Silva <2493377+askpt@users.noreply.github.com>

build/Common.props

@@ -6,6 +6,8 @@
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
     <!-- Workaround for IDE0005 (Remove unnecessary usings/imports); see https://github.com/dotnet/roslyn/issues/41640 -->
     <NoWarn>EnableGenerateDocumentationFile</NoWarn>
+    <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)'=='Debug'">

src/OpenFeature/Api.cs

@@ -54,7 +54,7 @@ public void SetProvider(FeatureProvider featureProvider)
         /// </summary>
         /// <remarks>The provider cannot be set to null. Attempting to set the provider to null has no effect.</remarks>
         /// <param name="featureProvider">Implementation of <see cref="FeatureProvider"/></param>
-        public async Task SetProviderAsync(FeatureProvider featureProvider)
+        public async Task SetProviderAsync(FeatureProvider? featureProvider)
         {
             this._eventExecutor.RegisterDefaultFeatureProvider(featureProvider);
             await this._repository.SetProvider(featureProvider, this.GetContext()).ConfigureAwait(false);
@@ -80,6 +80,10 @@ public void SetProvider(string clientName, FeatureProvider featureProvider)
         /// <param name="featureProvider">Implementation of <see cref="FeatureProvider"/></param>
         public async Task SetProviderAsync(string clientName, FeatureProvider featureProvider)
         {
+            if (string.IsNullOrWhiteSpace(clientName))
+            {
+                throw new ArgumentNullException(nameof(clientName));
+            }
             this._eventExecutor.RegisterClientFeatureProvider(clientName, featureProvider);
             await this._repository.SetProvider(clientName, featureProvider, this.GetContext()).ConfigureAwait(false);
         }
@@ -138,8 +142,8 @@ public FeatureProvider GetProvider(string clientName)
         /// <param name="logger">Logger instance used by client</param>
         /// <param name="context">Context given to this client</param>
         /// <returns><see cref="FeatureClient"/></returns>
-        public FeatureClient GetClient(string name = null, string version = null, ILogger logger = null,
-            EvaluationContext context = null) =>
+        public FeatureClient GetClient(string? name = null, string? version = null, ILogger? logger = null,
+            EvaluationContext? context = null) =>
             new FeatureClient(name, version, logger, context);
 
         /// <summary>
@@ -200,7 +204,7 @@ public void AddHooks(IEnumerable<Hook> hooks)
         /// Sets the global <see cref="EvaluationContext"/>
         /// </summary>
         /// <param name="context">The <see cref="EvaluationContext"/> to set</param>
-        public void SetContext(EvaluationContext context)
+        public void SetContext(EvaluationContext? context)
         {
             this._evaluationContextLock.EnterWriteLock();
             try

src/OpenFeature/Error/FeatureProviderException.cs

@@ -20,7 +20,7 @@ public class FeatureProviderException : Exception
         /// <param name="errorType">Common error types <see cref="ErrorType"/></param>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public FeatureProviderException(ErrorType errorType, string message = null, Exception innerException = null)
+        public FeatureProviderException(ErrorType errorType, string? message = null, Exception? innerException = null)
             : base(message, innerException)
         {
             this.ErrorType = errorType;

src/OpenFeature/Error/FlagNotFoundException.cs

@@ -15,7 +15,7 @@ public class FlagNotFoundException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public FlagNotFoundException(string message = null, Exception innerException = null)
+        public FlagNotFoundException(string? message = null, Exception? innerException = null)
             : base(ErrorType.FlagNotFound, message, innerException)
         {
         }

src/OpenFeature/Error/GeneralException.cs

@@ -15,7 +15,7 @@ public class GeneralException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public GeneralException(string message = null, Exception innerException = null)
+        public GeneralException(string? message = null, Exception? innerException = null)
             : base(ErrorType.General, message, innerException)
         {
         }

src/OpenFeature/Error/InvalidContextException.cs

@@ -15,7 +15,7 @@ public class InvalidContextException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public InvalidContextException(string message = null, Exception innerException = null)
+        public InvalidContextException(string? message = null, Exception? innerException = null)
             : base(ErrorType.InvalidContext, message, innerException)
         {
         }

src/OpenFeature/Error/ParseErrorException.cs

@@ -15,7 +15,7 @@ public class ParseErrorException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public ParseErrorException(string message = null, Exception innerException = null)
+        public ParseErrorException(string? message = null, Exception? innerException = null)
             : base(ErrorType.ParseError, message, innerException)
         {
         }

src/OpenFeature/Error/ProviderNotReadyException.cs

@@ -15,7 +15,7 @@ public class ProviderNotReadyException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public ProviderNotReadyException(string message = null, Exception innerException = null)
+        public ProviderNotReadyException(string? message = null, Exception? innerException = null)
             : base(ErrorType.ProviderNotReady, message, innerException)
         {
         }

src/OpenFeature/Error/TargetingKeyMissingException.cs

@@ -15,7 +15,7 @@ public class TargetingKeyMissingException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public TargetingKeyMissingException(string message = null, Exception innerException = null)
+        public TargetingKeyMissingException(string? message = null, Exception? innerException = null)
             : base(ErrorType.TargetingKeyMissing, message, innerException)
         {
         }

src/OpenFeature/Error/TypeMismatchException.cs

@@ -15,7 +15,7 @@ public class TypeMismatchException : FeatureProviderException
         /// </summary>
         /// <param name="message">Exception message</param>
         /// <param name="innerException">Optional inner exception</param>
-        public TypeMismatchException(string message = null, Exception innerException = null)
+        public TypeMismatchException(string? message = null, Exception? innerException = null)
             : base(ErrorType.TypeMismatch, message, innerException)
         {
         }

src/OpenFeature/EventExecutor.cs

@@ -14,7 +14,7 @@ internal class EventExecutor : IAsyncDisposable
     {
         private readonly object _lockObj = new object();
         public readonly Channel<object> EventChannel = Channel.CreateBounded<object>(1);
-        private FeatureProvider _defaultProvider;
+        private FeatureProvider? _defaultProvider;
         private readonly Dictionary<string, FeatureProvider> _namedProviderReferences = new Dictionary<string, FeatureProvider>();
         private readonly List<FeatureProvider> _activeSubscriptions = new List<FeatureProvider>();
 
@@ -99,7 +99,7 @@ internal void RemoveClientHandler(string client, ProviderEventTypes type, EventH
             }
         }
 
-        internal void RegisterDefaultFeatureProvider(FeatureProvider provider)
+        internal void RegisterDefaultFeatureProvider(FeatureProvider? provider)
         {
             if (provider == null)
             {
@@ -115,7 +115,7 @@ internal void RegisterDefaultFeatureProvider(FeatureProvider provider)
             }
         }
 
-        internal void RegisterClientFeatureProvider(string client, FeatureProvider provider)
+        internal void RegisterClientFeatureProvider(string client, FeatureProvider? provider)
         {
             if (provider == null)
             {
@@ -124,7 +124,7 @@ internal void RegisterClientFeatureProvider(string client, FeatureProvider provi
             lock (this._lockObj)
             {
                 var newProvider = provider;
-                FeatureProvider oldProvider = null;
+                FeatureProvider? oldProvider = null;
                 if (this._namedProviderReferences.TryGetValue(client, out var foundOldProvider))
                 {
                     oldProvider = foundOldProvider;
@@ -136,7 +136,7 @@ internal void RegisterClientFeatureProvider(string client, FeatureProvider provi
             }
         }
 
-        private void StartListeningAndShutdownOld(FeatureProvider newProvider, FeatureProvider oldProvider)
+        private void StartListeningAndShutdownOld(FeatureProvider newProvider, FeatureProvider? oldProvider)
         {
             // check if the provider is already active - if not, we need to start listening for its emitted events
             if (!this.IsProviderActive(newProvider))
@@ -174,7 +174,7 @@ private bool IsProviderActive(FeatureProvider providerRef)
             return this._activeSubscriptions.Contains(providerRef);
         }
 
-        private void EmitOnRegistration(FeatureProvider provider, ProviderEventTypes eventType, EventHandlerDelegate handler)
+        private void EmitOnRegistration(FeatureProvider? provider, ProviderEventTypes eventType, EventHandlerDelegate handler)
         {
             if (provider == null)
             {
@@ -202,22 +202,22 @@ private void EmitOnRegistration(FeatureProvider provider, ProviderEventTypes eve
                 {
                     handler.Invoke(new ProviderEventPayload
                     {
-                        ProviderName = provider.GetMetadata()?.Name,
+                        ProviderName = provider.GetMetadata().Name,
                         Type = eventType,
                         Message = message
                     });
                 }
                 catch (Exception exc)
                 {
-                    this.Logger?.LogError("Error running handler: " + exc);
+                    this.Logger.LogError(exc, "Error running handler");
                 }
             }
         }
 
-        private async void ProcessFeatureProviderEventsAsync(object providerRef)
+        private async void ProcessFeatureProviderEventsAsync(object? providerRef)
         {
-            var typedProviderRef = (FeatureProvider)providerRef;
-            if (typedProviderRef.GetEventChannel() is not { Reader: { } reader })
+            var typedProviderRef = (FeatureProvider?)providerRef;
+            if (typedProviderRef?.GetEventChannel() is not { Reader: { } reader })
             {
                 return;
             }
@@ -249,7 +249,7 @@ private async void ProcessEventAsync()
                     case Event e:
                         lock (this._lockObj)
                         {
-                            if (this._apiHandlers.TryGetValue(e.EventPayload.Type, out var eventHandlers))
+                            if (e.EventPayload?.Type != null && this._apiHandlers.TryGetValue(e.EventPayload.Type, out var eventHandlers))
                             {
                                 foreach (var eventHandler in eventHandlers)
                                 {
@@ -260,11 +260,11 @@ private async void ProcessEventAsync()
                             // look for client handlers and call invoke method there
                             foreach (var keyAndValue in this._namedProviderReferences)
                             {
-                                if (keyAndValue.Value == e.Provider)
+                                if (keyAndValue.Value == e.Provider && keyAndValue.Key != null)
                                 {
                                     if (this._clientHandlers.TryGetValue(keyAndValue.Key, out var clientRegistry))
                                     {
-                                        if (clientRegistry.TryGetValue(e.EventPayload.Type, out var clientEventHandlers))
+                                        if (e.EventPayload?.Type != null && clientRegistry.TryGetValue(e.EventPayload.Type, out var clientEventHandlers))
                                         {
                                             foreach (var eventHandler in clientEventHandlers)
                                             {
@@ -288,7 +288,7 @@ private async void ProcessEventAsync()
                                     // if there is an association for the client to a specific feature provider, then continue
                                     continue;
                                 }
-                                if (keyAndValues.Value.TryGetValue(e.EventPayload.Type, out var clientEventHandlers))
+                                if (e.EventPayload?.Type != null && keyAndValues.Value.TryGetValue(e.EventPayload.Type, out var clientEventHandlers))
                                 {
                                     foreach (var eventHandler in clientEventHandlers)
                                     {
@@ -311,7 +311,7 @@ private void InvokeEventHandler(EventHandlerDelegate eventHandler, Event e)
             }
             catch (Exception exc)
             {
-                this.Logger?.LogError("Error running handler: " + exc);
+                this.Logger.LogError(exc, "Error running handler");
             }
         }
 
@@ -325,7 +325,7 @@ public async Task Shutdown()
 
     internal class Event
     {
-        internal FeatureProvider Provider { get; set; }
-        internal ProviderEventPayload EventPayload { get; set; }
+        internal FeatureProvider? Provider { get; set; }
+        internal ProviderEventPayload? EventPayload { get; set; }
     }
 }

src/OpenFeature/Extension/EnumExtensions.cs

@@ -9,7 +9,7 @@ internal static class EnumExtensions
         public static string GetDescription(this Enum value)
         {
             var field = value.GetType().GetField(value.ToString());
-            var attribute = field.GetCustomAttributes(typeof(DescriptionAttribute), false).FirstOrDefault() as DescriptionAttribute;
+            var attribute = field?.GetCustomAttributes(typeof(DescriptionAttribute), false).FirstOrDefault() as DescriptionAttribute;
             return attribute?.Description ?? value.ToString();
         }
     }

src/OpenFeature/FeatureProvider.cs

@@ -45,7 +45,7 @@ public abstract class FeatureProvider
         /// <param name="context"><see cref="EvaluationContext"/></param>
         /// <returns><see cref="ResolutionDetails{T}"/></returns>
         public abstract Task<ResolutionDetails<bool>> ResolveBooleanValue(string flagKey, bool defaultValue,
-            EvaluationContext context = null);
+            EvaluationContext? context = null);
 
         /// <summary>
         /// Resolves a string feature flag
@@ -55,7 +55,7 @@ public abstract Task<ResolutionDetails<bool>> ResolveBooleanValue(string flagKey
         /// <param name="context"><see cref="EvaluationContext"/></param>
         /// <returns><see cref="ResolutionDetails{T}"/></returns>
         public abstract Task<ResolutionDetails<string>> ResolveStringValue(string flagKey, string defaultValue,
-            EvaluationContext context = null);
+            EvaluationContext? context = null);
 
         /// <summary>
         /// Resolves a integer feature flag
@@ -65,7 +65,7 @@ public abstract Task<ResolutionDetails<string>> ResolveStringValue(string flagKe
         /// <param name="context"><see cref="EvaluationContext"/></param>
         /// <returns><see cref="ResolutionDetails{T}"/></returns>
         public abstract Task<ResolutionDetails<int>> ResolveIntegerValue(string flagKey, int defaultValue,
-            EvaluationContext context = null);
+            EvaluationContext? context = null);
 
         /// <summary>
         /// Resolves a double feature flag
@@ -75,7 +75,7 @@ public abstract Task<ResolutionDetails<int>> ResolveIntegerValue(string flagKey,
         /// <param name="context"><see cref="EvaluationContext"/></param>
         /// <returns><see cref="ResolutionDetails{T}"/></returns>
         public abstract Task<ResolutionDetails<double>> ResolveDoubleValue(string flagKey, double defaultValue,
-            EvaluationContext context = null);
+            EvaluationContext? context = null);
 
         /// <summary>
         /// Resolves a structured feature flag
@@ -85,7 +85,7 @@ public abstract Task<ResolutionDetails<double>> ResolveDoubleValue(string flagKe
         /// <param name="context"><see cref="EvaluationContext"/></param>
         /// <returns><see cref="ResolutionDetails{T}"/></returns>
         public abstract Task<ResolutionDetails<Value>> ResolveStructureValue(string flagKey, Value defaultValue,
-            EvaluationContext context = null);
+            EvaluationContext? context = null);
 
         /// <summary>
         /// Get the status of the provider.

src/OpenFeature/Hook.cs

@@ -29,7 +29,7 @@ public abstract class Hook
         /// <typeparam name="T">Flag value type (bool|number|string|object)</typeparam>
         /// <returns>Modified EvaluationContext that is used for the flag evaluation</returns>
         public virtual Task<EvaluationContext> Before<T>(HookContext<T> context,
-            IReadOnlyDictionary<string, object> hints = null)
+            IReadOnlyDictionary<string, object>? hints = null)
         {
             return Task.FromResult(EvaluationContext.Empty);
         }
@@ -42,7 +42,7 @@ public virtual Task<EvaluationContext> Before<T>(HookContext<T> context,
         /// <param name="hints">Caller provided data</param>
         /// <typeparam name="T">Flag value type (bool|number|string|object)</typeparam>
         public virtual Task After<T>(HookContext<T> context, FlagEvaluationDetails<T> details,
-            IReadOnlyDictionary<string, object> hints = null)
+            IReadOnlyDictionary<string, object>? hints = null)
         {
             return Task.CompletedTask;
         }
@@ -55,7 +55,7 @@ public virtual Task After<T>(HookContext<T> context, FlagEvaluationDetails<T> de
         /// <param name="hints">Caller provided data</param>
         /// <typeparam name="T">Flag value type (bool|number|string|object)</typeparam>
         public virtual Task Error<T>(HookContext<T> context, Exception error,
-            IReadOnlyDictionary<string, object> hints = null)
+            IReadOnlyDictionary<string, object>? hints = null)
         {
             return Task.CompletedTask;
         }
@@ -66,7 +66,7 @@ public virtual Task Error<T>(HookContext<T> context, Exception error,
         /// <param name="context">Provides context of innovation</param>
         /// <param name="hints">Caller provided data</param>
         /// <typeparam name="T">Flag value type (bool|number|string|object)</typeparam>
-        public virtual Task Finally<T>(HookContext<T> context, IReadOnlyDictionary<string, object> hints = null)
+        public virtual Task Finally<T>(HookContext<T> context, IReadOnlyDictionary<string, object>? hints = null)
         {
             return Task.CompletedTask;
         }